1. Technical Field
The disclosed embodiments generally relate to the field of power and impedance measurement.
2. Description of the Related Art
In a typical RF plasma generator arrangement, a high-power RF source (RF power generator) produces an RF signal at a known frequency that is supplied along a power conduit to a plasma chamber. Typically, a severe impedance mismatch between the RF power generator and the plasma chamber is present. Accordingly, an impedance matching network is conventionally interposed between the RF power generator and the plasma chamber.
Because of non-linearities in the plasma chamber and losses in the line and/or in the impedance matching network, the complete RF signal does not reach the plasma chamber. As such, a probe is conventionally employed at the power input to the plasma chamber to detect the voltage and current of the RF signal. By accurately measuring the voltage and current, a user can obtain a better indication of the quality of the plasma and better control etching characteristics for a silicon wafer or other device in the plasma chamber.
Conventional methods of detecting the voltage and current of the RF wave have been proposed. For example, U.S. Reissued Patent Number RE 38,273 to Gerrish et al. describes the use of a sampling probe at the input to a plasma chamber for detecting plasma RF voltage and current. The voltage and current RF signals are mixed with another signal, such as a signal differing from the main signal by between 0.2 KHz and 20 KHz, to produce a heterodyned signal. The heterodyned signal is then processed using a Fast Fourier transform (FFT) algorithm to calculate voltage and current magnitudes and relative phase angle information.
U.S. Pat. No. 5,565,737 to Keane describes the use of an aliasing sampler probe at the input to the plasma chamber for detecting plasma RF voltage and current. The sampler probe uses a sampling signal with a sampling frequency that is slower than the RF fundamental frequency. The sampling frequency is determined by summing the fundamental frequency with an aliasing frequency that is independent of the fundamental frequency and dividing the sum by a positive integer.
One problem with the above-described methods and systems is that the signal generated by the RF power generator can severely degrade due to an impedance mismatch between the RF power generator (or other components) and the plasma chamber. Another problem occurs when an impedance matching network is used between the output of the RF power generator and the input of the plasma chamber. In this case, the RF power-delivered to the plasma chamber can vary due to either adjustments within the impedance matching network and/or losses in the components and circuitry connected between the output of the RF power generator and the input of the plasma chamber. As such, the output of the RF power generator may become unstable due to interaction between the control loop of the RF power generator and the control loop of the impedance matching network. In addition, the use of an aliasing frequency that is independent of the RF fundamental frequency can result in a non-uniform sampled signal.
Other proposed systems include that described in U.S. Pat. No. 6,046,594 to Mavretic describes a method and apparatus for measuring electrical characteristics (e.g. current, voltage, phase, etc.) between a power source and a load at a set of harmonic frequencies to determine information about the load (e.g., load impedance, power dissipation, etc). As described therein, a first circuit detects a set of electrical characteristics of a signal between the power source and the load; a second circuit provides data representing the set of electrical characteristics at a harmonic frequency associated with the signal; and a third circuit receives the data and determines information about the load at the harmonic frequency. However, such a system requires filtering at a particular harmonic frequency. While this may provide information at the particular harmonic frequency, information at the fundamental frequency or other harmonic frequencies may not be obtainable using such a system.
Accordingly, what is needed is a method and system for sampling a detected signal at a location where the signal is tightly coupled to the RF output of the RF power generator and is less affected by the losses in the RF path between the RF power generator and the input of the plasma chamber.
A need exists for a method and system for sampling a detected signal at a sampling frequency that is dependent upon the fundamental frequency of a detected signal.
A further need exists for a method and system for sampling a detected signal such that information pertaining to the fundamental frequency and one or more harmonics is obtainable from the sampling process.
The present disclosure is directed to solving one or more of the above-listed problems.